Activins are dimeric proteins structurally similar to inhibin, TGF-.beta.1, TGF-.beta.2, and other proteins that makeup a family of proteins structurally related to TGF-.beta.1. These proteins exhibit the chromatographic properties of TGF-.beta.s. In addition to having homology with respect to the amino acid sequences, activins exhibit conservation of cysteine positions characteristic of the TGF-.beta.s. Activins exhibit a molecular weight of 25 kD under nonreducing conditions by SDS-PAGE (and a molecular weight of 14 kD under reducing conditions). There are two known forms of the activin subunits, which have been termed .beta.A or .beta.B. Homodimeric forms .beta.AA and .beta.BB and a heterodimeric form .beta.AB have been described in the literature. Activin subunits have about a 30% homology to TGF-.beta.1 and TGF-.beta.2 chains in terms of their amino acid sequences. Inhibins are polypeptides which are also structurally related to activins. Inhibins are heterodimers of the activin .beta.A or .beta.B subunit and a separate .alpha. subunit. Inhibins exhibit activity essentially opposite to activin.
The activin .beta.A homodimer and .beta.AB heterodimer have been purified from porcine ovarian follicular fluid, and have been shown to stimulate the release of follicle stimulating hormone (FSH) from rat pituitary cells in vitro (W. Vale et al., Nature (1986) 321:776-79). Other reported activities include stimulation of oxytocin release from neurosecretory neurons (P. E. Sawchemko, et al., Nature (1988) 334:615-17; W. Vale et al., "Recent progress in Hormone Research" (1988) 44:1-34); stimulation of insulin secretion from pancreatic islets (Y. Totsuka et al., Biochem. & Biophys. Res. Comm. (1988) 156:335-39); and stimulation of erythroid and multi-potential progenitor cell colony formation in bone marrow culture (J. Yu et al., Nature (1987) 330:765-67; H. E. Broxmeyer et al., Proc. Natl. Acad. Sci. U.S.A. (1988) 85:9052-56). Activin .beta.A is apparently identical to erythroid differentiation factor (EDF) (M. Murata et al., Proc. Natl. Acad. Sci. U.S.A. (1988) 85:2434-38).
Despite the fact that activin is similar in amino acid sequence to TGF-.beta., activin does not compete with TGF-.beta. for binding to TGF-.beta. receptors types I, II, or III present on fibroblasts and epithelial cells. However, activin has been reported to compete against binding of TGF-.beta.1 to rat pituitary tumor cells (S. Cheifetz et al., J. Biol. Chem. (1988) 263:17225-28). TGF-.beta.1 and TGF-.beta.2 have been reported to induce formation of endochondral bone in vivo (M. E. Joyce et al., J. Cell Biol. (1990) 110:2195-2207, H. Bentz, et al. (1989) J. Biol. Chem., 264:20805-10).
Although the mRNA encoding activin .beta.A has been detected in several different tissues, including placenta, pituitary, bone marrow, kidney, spinal cord and brain (H. Meunier et al., Proc. Natl. Acad. Sci U.S.A. (1988) 85:247-51), to date the protein has been isolated only from porcine ovarian follicular fluid. What has not been reported is the isolation and purification of activin from bone and its ability to induce bone growth and maturation. Thus, there remains a need for the development of methodologies to extract activin from bone and to develop compositions and treatment modalities to induce bone growth and maturation. The present invention offers such.